Differential secretion mechanisms and predicted structural flexibility are described as additional effects of MICA polymorphism in the immune response. Structural features might explain impaired antibody recognition of certain MICA alleles, supporting that MICA genotyping should be considered in biomarker studies and NKG2D-based therapeutic development.
ABSTRACT
NKG2D is an essential receptor for cancer immunosurveillance, activating Natural Killer (NK) and CD8+ T-cells to eliminate transformed cells. However, release of its ligands promotes immune evasion. Although new therapeutic strategies aim to block NKG2D-ligand shedding, the impact of allelic polymorphism on secretion mechanisms, either as soluble proteins or within extracellular vesicles (EVs), remains insufficiently explored. The glycosyl–phosphatidyl–inositol (GPI)-anchored MICA (MHC class I chain-related gene A) *008 allele is preferentially incorporated into EVs, but little is known about the EV release of other alleles. Here, we compared five MICA alleles and found that non-GPI-MICA can also be incorporated into EVs through a palmitoylation-dependent process, although with much lower efficiency compared with GPI-anchored alleles, such as MICA*008. EVs containing MICA decreased NKG2D detection and impaired NK cell function, whereas soluble MICA had negligible effects. The strength of these effects was related to MICA allele identity. In addition to affinity (ruled by dimorphism at position 129), predictions of differences in MICA structural dynamics and NKG2D binding energetics coincided with the differential capacity of alleles to modulate NKG2D and to bind certain anti-MICA antibodies. Given the clinical potential of MICA as a biomarker and therapeutic target, defining the biology and immune impact of allelic variants is essential to optimize strategies aimed at restoring NKG2D-dependent antitumor immunity.